1. Field of the Invention
The present invention relates to a method and apparatus for the combustion treatment of a waste gas containing toxic gas from electronics and semiconductor industries for detoxicating such waste gas. More particularly, the present invention relates to a method and apparatus for the combustion treatment of a waste gas containing a toxic gas which, on combustion, is oxidized to form microparticles. The waste gas is subjected to a combustion treatment in a furnace where the toxic gas is oxidatively burnt and the resultant microparticles are absorbed in an aqueous film or droplets, and is then discharged out of the furnace together with water absorbing the microparticles without permitting the formation of any deposit of the microparticles on the inner wall of the furnace.
2. Description of the Prior Art:
In electronics and semiconductor industries, a waste gas containing gaseous toxic substances represented by arsine (AsH.sub.3), phosphine (PH.sub.3), diborane (B.sub.2 H.sub.6), monosilane (SiH.sub.4), etc. is formed in the steps for manufacturing semi-conductors. Since such waste gas is highly toxic to humans, complete elimination of such toxic substances contained in the waste gas is necessary prior to exhaust of the waste gas to atmosphere.
Various methods are known for effectively eliminating or detoxicating such toxic substances, such as a combustion method and an adsorption method. Among these methods, the adsorption method is easy in operation and thus employed conventionally to detoxicate the waste gas, adsorbed onto an adsorbent so that the waste gas thus treated can safely be exhausted from the production system. In this adsorption method, however, the used adsorbent contains the toxic substances and so has to be brought to a complicated secondary treatemnt for detoxicating it. Further, a lot of replacement or activation of the adsorbent are required during the operation. Accordingly, this adsorption method necessitates further treatment and replacements of adsorbent and so cannot be said to be economically suitable.
On the other hand, the combustion method contemplates oxidative decomposition of the toxic substances in the waste gas under combustion conditions whereby the gaseous toxic substances are oxidatively converted into solid microparticle in the form of oxides and can be removed from the gaseous substances. Such combustion method is known, for example, in Japanese Laid-open Patent Appln. Nos. Sho. 62-134414 and 62-152517. As no replacements are needed for the combustion method, in case of mass treatment of waste gas, the combustion method is superior.
Among the gaseous toxic substances, for example, arsine is oxidatively converted according to such a combustion method into arsenic oxide (As.sub.2 O.sub.3, As.sub.2 O.sub.5), phosphine into phosphorus pentoxide (P.sub.2 O.sub.5), and silane into silicon oxides (SiO, SiO.sub.2). The majority of these oxidized solid substances are also toxic and have to be eliminated completely from the combustion gas prior to exhaust to atmosphere. As these solid substances are formed in gaseous phase decomposition, the size of these particles is in the order of sub-micron, unlike ordinary particles of several microns which are, for example, contained in smoke dusts. Accordingly, it is extremely difficult to remove these microparticles completely from the combustion gas unlike the removal of larger particles. In the prior art combustion methods as mentioned above, air is used as a combustion-supporting gas for burning the toxic gas and a stream of air is blown into the combustion furnace along the wall thereof in order to protect the surface of the inner wall from deposit of such microparticles, simultaneously with forming an air flow of a high linear velocity towards an exit of the furnace to entrain the formed microparticles in the exhaust gas to be discharged out of the combustion furnace. The exhaust gas from the furnace is then introduced into a wet-type dust-removing apparatus installed outside the furnace where the microparticles are captured and separated from the combustion gas which can now be exhausted safely.
However, there are a number of drawbacks to be overcome also in such an air combustion method. As the waste gas has to be treated in two steps with a combustion furnace and a wet-type dust-removing apparatus, such a method is not efficient in operation and needs a large space for installation. Further, the removal rate of solid microparticles in the conventional wet-type dust-removing apparatus is inherently not so high. As the solid microparticles are captured outside the combustion furnace, the microparticles have to be discharged entirely out of the furnace together with the exhaust gas. However, it is extremely difficult to discharge the microparticles entirely from the furnace since they tend to deposit on the inner wall of the combustion furnace. In the prior art methods, therefore, it is proposed that a stream of air having a relatively high linear velocity is compulsorily formed in the furnace towards an exit thereof to prevent deposit of the microparticles on the inner wall of the furnace. The formation of a stream of air having such a high linear velocity is not only economically disadvantageous but still unsatisfactory to protect the inner wall completely from deposit of the microparticles. Thus, a small amount of the microparticles is permitted to deposit on the inner wall even in case of using a stream of air having a very high linear velocity. The amount of the microparticles deposited on the inner wall becomes larger evenly or locally with the lapse of time. Accordingly, a large amount of the deposit built up on the inner wall will eventually be dropped as a lump irregularly from time to time from the inner wall, and as a result of this phenomenon, the operating conditions of the combustion furnace, especially the pressure condition for combustion significantly fluctuate to the extent of preventing complete combustion of the toxic gas. What is more, deposit of the microparticles on the inner wall causes corrosion of the furnace.
In such air combustion methods, the extent of the flame stably existing in the furnace is extremely narrow relative to the flow rate of the waste gas so that the burner of the furnace tends to be blown off particularly in case the linear velocity of the air stream is increased with an attempt to enhance the effect of discharging the microparticles out of furnace. This phenomenon becomes significant especially in case of the combustion of arsine, phosphine or the like gas which is poor to form a flame on combustion, thus risking that the toxic gas will be discharged without being burnt or decomposed. If toxic gas still remains undecomposed in the exhaust gas from the combustion furnace, a secondary treatment such as an adsorption treatment becomes necessary to eliminate the toxic gas completely. As the volume of the exhaust gas is increased by using a stream of air having a high linear velocity, a large size dust-removing apparatus will be necessary to deal with the exhaust gas. When the oxidation in the combustion furnace is complete, the resultant microparticles consist of highly oxidized substances such as As.sub.2 O.sub.5, P.sub.2 O.sub.5 and SiO.sub.2. In case the oxidation in the combustion furnace is incomplete for the reasons as described above, however, the resultant microparticles will include partially oxidized or pyrolyzed substances. For example, the combustion treatment of AsH.sub.3 permits the formation of As.sub.2 O.sub.3 and As in addition to As.sub.2 O.sub.5. Likewise, the combustion treatment of PH.sub.3 also permits the formation of P.sub.2 O.sub.3 and P, and the combustion treatment of SiH.sub.4 also permits the formation of Si and SiO which is highly self-ignitable. These partially oxidized or pyrolyzed substances are insoluble or sparingly soluble in water and so are hardly removed from the exhaust gas by a wet-type dust-removing apparatus. Furthermore, self-ignitable substances such as SiO are hazardous which makes their handling difficult.
In the above mentioned prior art methods, the microparticles are discharged out of the furnace together with the combustion gas and then introduced into a wet-type dust-removing apparatus or an air-liquid separator for eliminating the microparticles. In this case, the wet-type dust-removing apparatus has a dual function of removing the microparticles and cooling the combustion gas and is generally selected from a spray tower, a packed bed and a venturi scrubber.
Thus, the prior art methods involve a number of problems to be solved, especially in complete elimination of the gaseous toxic substances by combustion and in prevention of the inner wall of the furnace from deposit of the microparticles. Accordingly, there is still a great demand to develop new method and apparatus for the combustion treatment of gaseous toxic substances wherein the various drawbacks found in the prior art methods are overcome.